>>30639744Gen V was basically a soft reboot meant for long time fans. Hence the large Pokedex, only having access to those Pokemon in BW, having direct sequels like Gold/Silver rather than something unrelated, plot that focuses on the morality of capturing and training Pokemon, etc.
They did this because they knew that as time was passing this would probably be the last chance to appeal to that crowd before they got too old and, more importantly, the playerbase got flooded with tons of children who needed to be appealed to in a totally different way. Kids don't care about difficulty. If anything it can be a deterrent when not done carefully. This is also the reason there have been less new Pokemon added in the last two Gens. For children that are new to the series and those who haven't been around from the start, the new Pokemon would be just as foreign as the old Pokemon. So instead there's a focus on making the older ones relevant while still introducing new ones, but at a slower rate than before.
tl;dr Game Freak added it and took it away because they're aware of changing demographics. And also because they're lazy and didn't wanna add it again to later games.
